Arrangement for correcting deflection errors in a cathode-ray tube



June 10, 1969 E. E. HIMMELBAUER ETAL 3,449,621

ARRANGEMENT FOR CORRECTING DEFLECTION ERRORS IN A CATHODE-RAY TUBE FiledAug. 10. 1966 Sheet of 2 F|G.2 c/ can INVENTORS ERICH E. HIMME-LBAUER BYJAN PEPER AGENT! n 1969 E. E. HIMMELBAUER ETAL 3,

ARRANGEMENT FOR CORRECTING DEFLECTION ERRORS IN A CATHODE-RAY TUBE FiledAug. 10, 1966- Sheet Z or 2 FIG. 3c

FIG.4

INVENTORS ERICH E. HIMMELBAUER JAN PEPER assur United States Patent US.Cl. 315-25 Claims ABSTRACT OF THE DISCLOSURE A cathode ray tube thatincludes electrostatic deflection means for deflecting the electron beamalong first and second perpendicular axes. Deflection errors arecompensated by providing means for producing a quadripolar magneticfield having asymptotes that are parallel to said axes. The quadripolarfield is located between the electrostatic deflection means and thescreen of the tube so as to rotate the deflection axes of the electronbeam.

The present invention relates to an arrangement for correctingdeflection errors in a cathode-ray tube in which the electron beam isdeflected by electrostatic deflection means in two directionssubstantially at right angles to each other.

The arrangement in accordance with the invention is characterized inthat, viewed in the travelling direction of the electrons, a quadrupolarmagnetic field is produced after said deflection means. The main axes ofthe quadrupolar field substantially coincide with the directions ofdeflection so that a rotation of said directions of deflection isachieved.

The invention is based on the recognition of the fact that orthogonalityerrors resulting from mechanical tolerances in the arrangement of thedeflection plates and from any deviations in the after-accelerationfield can be eliminated by means of such a quadrupolar magnetic field.

The invention will now be described more fully, by way of example, withreference to the accompanying figures, in which:

FIG. 1 shows an example of the supply, the structure and the location ofcoils for producing a quadrupolar magnetic field.

FIG. 2 shows a quadrupolar field produced by means of permanent magnets.

FIGS. 3a-3c illustrate a first coil configuration for use in thearrangement of FIG. 1.

FIG. 4 shows a second coil configuration.

FIG. 1 shows a cathode-ray tube 1 with having a screen 2 on which thedirections of deflection are substantially at right angles to each otherand are indicated by the lines AA and BB. Deflection in both directionsis obtained with the aid of deflection means the operation of which isbased on the electrostatic principle. A direct current source 3 suppliescurrent to the coils 7 and 8 through control members 4 and 5 and throughthe lead-in and lead-out wires 6. The coils 7 and 8, supplied withdirect current in a manner to be described hereinafter, produce aquadrupolar field. Such a field may also be produced, for example, byfour permanent magnets or electromagnets.

In FIG. 2, reference numeral 9 designates North poles and referencenumeral 10 denotes South poles of the magnets. The quadrupolar magneticfield is mainly determined by the magnetic field strength 11, thedirections Patented June 10, 1969 of which extend along the axes CC andDD. At the centre point of the quadrupolar field, that is to say at thepoint of intersection of these main axes, the field strength is zero ifthe strengths of the magnets in the axis CC and in the axis DD are equalto each other. The magnetic field strength increases substantiallylinearly from the centre point of the quadrupolar field towards themagnets. If the direction of the electron beam is assumed to goperpendicularly into the plane of the drawing, reference numeral 12designates the force exerted on the electrons in situ by the magneticfield strength 11. If an electron beam is deflected towards the axis CC,this deflection will be rotated under the influence of the quadrupolarmagnetic field in the direction of the axis C'C. Similarly, thedeflection along the axis DD will be displaced to the axis D'D'.

Consequently, it is found to be possible to displace the deflection ofthe electrons along the axes CC and DD to the axes C'C' and DD' by theuse of the quadrupolar magnetic field. If the axes of deflection AA andBB are at an angle of (-a) to each other, the angle a can be eliminatedwith the aid of the quadrupolar magnetic field if the angles between theaxes CC and CC and between the axes DD and DD together constitute theangle a so that the directions of deflection AA and BB on the screen 2are put at right angles to each other. Thus, according to the invention,orthogonality errors due to mechanical tolerances in the arrangement ofthe electrostatic deflection means and to any deviations in theafter-acceleration field can be corrected. For measuring purposes inoscillograph tubes, such orthogonality errors are often very disturbingso that, more particularly with the use of these tubes, it is desirablefor said errors to be eliminated. However, if desired, it is alsopossible to adjust angles different from 90 with the aid of such aquadrupolar field.

Sectional views of the coils 7 and 8 in a plane at right angles to theaxis of the cathode-ray tube 1 are shown in FIG. 3.

The directions of the currents in the coils are designated by a dot forthe current coming out of the plane of the drawing and by a cross forthe current going into the plane of the drawing. Currents of equalstrengths in the coils 7 and 8 having the directions indicated in FIG.3a produce the quadrupolar magnetic field already describedhereinbefore. Currents of equal strengths in the coils 7 and 8 havingthe directions indicated in FIG. 3b produce a normal bipolar magneticfield which may be used as a deflection field. If the strengths of thecurrents in the coils 7 and 8 of FIG. 3a are different, for example, inthe coil 7 one unit current and in the coil 8 three units current, thecurrent in the coil 7 may have the course shown in FIG. Be. It can nowbe readily understood that a quadrupolar field is produced by two unitscurrent (shown together in FIG. 30) while a bipolar deflection field isproduced by one unit current (shown separately in FIG. 3c). Thequadrupolar field provides the desired orthogonality correction, whilethe bipolar deflection field may be used for centering the image on thescreen 2 in one direction. Consequently, the average field strength ofthe resulting field is greater in the direction of the bipolar fieldthan in the direction at right angles thereto.

If two units current are applied to the coil 8 while the coil 7 does notreceive current, this results in that in the manner described above oneunit current produces a quadrupolar magnetic field while the other unitproduces a bipolar magnetic field. Consequently, if desired, the coil 7could be omitted so that the coil 8 alone provides both an orthogonalitycorrection and a centering of the image in one direction of deflection.The correction of an orthogonality error of 1 to 2 then results in anegligible displacement of the image.

If, beside the desired angular rotation, a centering of the image on thescreen 2 in both directions of deflection brought about by theelectrostatic deflection means should be required, use may be made ofthe coil configuration shown in FIG. 4. The coils 7 and 8 in which thecurrents have the directions shown in FIG. 4 and different strengths maybe used for the angular rotation of both directions of deflection andfor the centering of the image in one direction of deflection of theelectrostatic deflection means, While the coils 13 and 14 in which thecurrents have the indicated directions and equal strengths may bringabout the centering of the image in the direction of deflection at rightangles to the above direction of deflection.

It should be appreciated that an increase of the average field strengthin one of the directions with respect to this average field strength inthe other direction may also be achieved, for example, by the use of twodiflerent permanent magnets located diametrically opposite one anotherin an arrangement shown in FIG. 2.

Moreover, it should be appreciated that an orientation of the bipolarfield with respect to the quadrupolar field different from thatdescribed hereinbefore may give rise to a displacement of the imagewhich does not coincide with one of the directions of deflection.

What is claimed is:

1. An arrangement for correcting deflection errors in a cathode ray tubehaving an electron beam source and a screen for receiving the electronbeam comprising, electrostatic deflection means positioned along thebeam path for deflecting said beam in two directions substantially atright angles to each other, and means positioned along the beam pathbetween said electrostatic deflection means and said screen forproducing a quadrupolar magnetic field in said tube having asymptotesthat are substantially parallel to said directions of deflection toproduce a rotation of said directions of deflection of the electronbeam.

2. An arrangement as claimed in claim 1 wherein said magnetic fieldproducing means includes means for superimposing a bipolar magneticfield on said quadrupolar field thereby to displace the image formed onsaid screen.

3. An arrangement as claimed in claim 1 wherein said magnetic fieldproducing means comprises first and second coils mounted on said tubeand means for applying thereto DC currents that are adjustable inamplitude and direction.

4. A cathode ray tube comprising, an electron gun, a screen forreceiving the electron beam produced by said gun, electrostaticdeflection means positioned between said gun and screen for deflectingthe electron beam along first and second substantially perpendicularaxes, and means for producing a quadrupolar magnetic field in said tubehaving a pair of asymptotes that are perpendicular to the electron beamand coincide with said first and second axes, said field producing meansbeing positioned between said electrostatic deflection means and saidscreen thereby to produce a rotation of the electron beam.

5. A cathode ray tube as claimed in claim 4 wherein said magnetic fieldproducing means comprises, first and second coils mounted on said tube,a source of DC current coupled to said coils, and means for individuallyadjusting the amplitudes of the currents supplied to said coils.

References Cited UNITED STATES PATENTS 2,102,421 12/1937 Kuehni 313-792,188,579 1/ 1940 Schlesinger 3l379 2,212,640 8/1940 Hogan 31379 X2,454,345 11/1948 Riidenberg 5 313--79 2,795,717 6/1957 Finkelstein etal. 315--27 X 2,889,547 6/1959 Wesley 3l527 X RODNEY D. BENNETT, JR.,Primary Examiner.

CHARLES L. WHITHAM, Assistant Examiner.

US. Cl. X.R. 313-79; 315-27

